The electronic structure of a semiconductor material can be investigated by photoluminescence imaging or photoluminescence spectroscopy techniques. In such techniques, a laser is directed onto a semiconductor sample, such as single crystalline silicon. If a photon emitted from the laser has energy greater than the band gap energy of the sample, then it can be absorbed by the semiconductor and excite electrons directly from the full valence band to the empty conduction band, leaving behind electron “holes” in the valence band. Delocalized pairing of free electrons in the conduction band with holes in the valence band gives rise to electron-hole pairs. If electrostatic forces within an electron-hole pair are sufficient to cause binding of the electron to the hole, an exciton is formed. At low temperatures, non-equilibrium electrons, holes, and excitons can stably bind into a plasma that can undergo condensation to form an electron-hole droplet (EHD).
Excited electrons in the conduction band can return to equilibrium by losing their excess energy and recombining with the valence band. This process of electron-hole pair/exciton annihilation can be studied to yield information about a semiconductor sample, including information about impurities that may be contained therein. In one type of recombination event, exciton decay and recombination occurs at a free lattice site (free exciton) and results in the spontaneous emission of a photon having a wavelength corresponding to the energy released. The released photon is observable by photoluminescence analysis, the spectrum of which provides information about the intrinsic semiconductor.
In another type of recombination event, the decaying electron can become loosely bound at an impurity that provides donor or acceptor bands that are within the band gap energy but are distinct from the valence and conduction bands of the intrinsic semiconductor material. The impurity band can absorb the energy associated with the initial decay of the electron from the conduction band to the impurity band and dissipate it as a phonon. Electrostatic forces between the bound electron and a free hole can cause binding of the electron to the hole to form a bound exciton. Decay of the exciton and recombination of the electron with the valence band is associated with emission of a photon that can be observed by photoluminescence analysis, the resulting spectrum providing information characteristic of the impurity. For example, the shape and intensity of the bound exciton peak can be used to determine the impurity density.
At low temperature, photoluminescence spectra show distinct recombination phenomena—recombination of a bound electron in the impurity band with a free hole, giving rise to a bound exciton peak (BE peak), and recombination of a free electron in the intrinsic semiconductor with a free hole, giving rise to a free exciton peak (FE peak). The wavelength of observed photon emissions depends on the type of recombination center within the sample and whether or not phonons are associated with the recombination process. Most peaks of interest are phonon-assisted emissions, wherein intensity at a given wavelength is related to the number of sites within the sample that are responsible for that emission. Because concentration of an electrically active impurity is proportional to the ratio of the intensities of the BE and FE peaks, photoluminescence analysis is a powerful method for the non-destructive analysis of shallow impurities within a sample.
While photoluminescence analysis is a powerful method for analysis of impurities, its accuracy and sensitivity are complicated by peaks associated with the EHD. Exciton density within a sample increases with increasing intensity of the incident laser, and once a critical exciton density is reached, the EHD can form. Recombination events within the EHD give off characteristic luminescence that results in a broad peak underlying and interfering with the BE and FE peaks in the transverse optical (TO) phonon region of the spectrum (8757-8889 cm−1/1142-1125 nm). Moreover, because the EHD peak intensifies and shifts to lower energies with increasing excitation intensity, the interference becomes more pronounced with increasing laser power. Thus, it is often difficult to accurately determine the ratio of BE to FE emissions due to interference from the EHD peak, particularly interference between the EHD and BE peaks. Conventional methods of overcoming the difficulties in resolving the BE and FE peaks from the EHD peaks often involve varying excitation energy, such as operating a laser at lower excitation energies, use of extended data collection periods, use of differing laser wavelengths, or a combination thereof. For example, one conventional approach is to run an instrument at low excitation energies in order to minimize the EHD effect. However, this approach reduces the signal-to-noise in the BE and FE emissions, which makes it difficult to effectively measure impurities in high purity semiconductor materials. Another conventional example for measuring semiconductor materials with low levels of impurities and avoiding the EHD is to take longer instrument scans. However, this approach can be a hindrance for efficiently testing large numbers of samples.
In the silicon industry, two principal types of instruments are used for photoluminescence analysis of impurities—dispersive infrared instruments operating under high sample excitation conditions, and Fourier transform instruments operating under low sample excitation conditions. As practiced, both techniques involve “straight-on” collection of emitted photons from silicon samples, wherein the sample is oriented at a position that is parallel to the collection optics. In this orientation, the silicon sample is illuminated by the laser and the emitted photons are collected from the front surface of the sample by a collection lens having a lens focal point at the front surface.
In summary, conventional photoluminescence analysis can be generalized as having four primary steps: (i) excitation of the front surface of a semiconductor sample with a laser at low temperature, said laser operating at either a fixed or variable power to achieve resolution of BE and FE peaks from EHD peaks in resultant spectra; (ii) emission of photons from the sample, said photons being characteristic of BE, FE, and EHD recombination events; (iii) detection of photons emitted from the front surface of the sample by collection optics that are oriented parallel to the sample; and (iv) analysis of the resultant spectra to determine information about the impurities. Such conventional methods and apparatuses are highly sensitive and can typically identify impurity densities of low parts-per-trillion (“ppt”) atoms within the crystal lattice. Nevertheless, there remains a need in the art for more sensitive techniques and apparatuses for identifying and quantifying impurities in semiconductor materials.